1. Field of the Invention
This invention relates generally to a corona igniter for emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge.
2. Description of the Prior Art
Corona discharge ignition systems provide an alternating voltage and current, reversing high and low potential electrodes in rapid succession which makes arc formation difficult and enhances the formation of corona discharge. The system includes a corona igniter with a central electrode charged to a high radio frequency voltage potential and creating a strong radio frequency electric field in a combustion chamber. The electric field emitted from the central electrode causes a portion of a mixture of fuel and air to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
The central electrode of the corona igniter is formed of an electrically conductive material, which receives the high radio frequency voltage and emits the radio frequency electric field into the combustion chamber to ionize the fuel-air mixture and provide the corona discharge. An insulator formed of an electrically insulating material surrounds the central electrode and is received in a metal shell. An example of a corona igniter is disclosed in U.S. Patent Application Publication No. US 2010/0083942 to the present inventor, Lykowski. The igniter of the corona discharge ignition system does not include any grounded electrode element intentionally placed in close proximity to a firing end of the central electrode. Rather, the ground is provided by a piston disposed in the combustion chamber below the corona igniter, or by walls of a cylinder block and cylinder head surrounding the corona igniter and forming the combustion chamber.
The intensity of the electric field emitted from the corona igniter is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge, also referred to as a non-thermal plasma, occurs at the central electrode firing end, rather than a thermal plasma or electric arc. The corona discharge provided by the central electrode is also preferably concentrated in a predetermined direction to provide a strong ignition of the fuel-air mixture. However, since the electric field is attracted to the grounded piston, cylinder block, and cylinder head, the corona discharge spreads in many directions, which limits the quality of ignition.